A continuous annealing furnace, as is well known, is able to heat, soak and cool a steel strip continuously, and when required, to subsequently apply overaging treatment to it. In these processes, besides the temperature of the heating (annealing temperature) and the time of the soaking, cooling a steel strip is important to obtain a steel strip having the desired properties. For instance, in order to enhance the aging property, fluting resistance and other properties of a steel strip, increasing the rate of the cooling and then applying the overaging treatment is believed to be effective. A variety of cooling medium are currently used for cooling a steel strip after the heating and soaking, and the rate of cooling a steel strip is different depending on the choice of the cooling medium.
A very high cooling rate can be obtained when water is used as the cooling medium; a cooling rate in the range of ultra rapid cooling can be attained. The most serious drawback of the water cooling is, however, that a strip deformation called cooling buckle occurs as a result of quenching strain. Another problem is that an oxide film forms on the surface of a strip owing to the contact with water, and an additional facility to remove the oxide film is necessary. For these reasons, a water cooling apparatus is economically disadvantageous.
As a means to solve the above problem, a roll cooling method, wherein a steel strip is cooled by making it contact the surface of a roll cooled by water or some other cooling medium circulating through it, is employed. This method, however, has the following problem.
All the steel strips passing through a continuous annealing furnace are not necessarily flat and, therefore, there are cases that the strip contacts the cooling roll only partially across the width. The local lack of contact causes uneven cooling of the strip in the transverse direction, resulting in the deformation of the steel strip. This necessitates a means to make the strip flat before contacting the cooling roll, which increases equipment costs.
As another cooling means, a cooling method using a gas as a cooling medium has been commercially applied, and there are various records of this method. While the cooling rate by this method is lower than the water cooling or the roll cooling mentioned above, it enables comparatively uniform cooling in the transverse direction. For the purpose of raising the cooling rate, which constitutes the most serious shortcoming of the gas cooling method, a technique to raise the cooling rate by disposing the tips of the nozzles for blowing the cooling medium gas as close to the steel strip as possible and thus raising the rate of heat conduction and another to use hydrogen gas as the blown gas have been disclosed.
Japanese Examined Patent Publication No. H2-16375 is an example of the technique to raise the heat conductivity by disposing the tips of the gas blowing nozzles close to the steel strip. This is a technology to realize efficient cooling by decreasing the distance from the nozzle tips to the steel strip. In the proposed technology, specifically, the length of the nozzles protruding from a surface of a cooling gas chamber (cooling box) is set at 100 mm−Z or more (where Z is the distance from the nozzle tips to the surface of the steel strip) and, by this, a chamber is provided for the gas blown through the protruding nozzles to flow backward after hitting the steel strip. Said publication discloses that this arrangement decreases the stagnation of the blown gas at the steel strip surface and enhances the cooling uniformity in the strip width direction.
Further, they carried out an experiment to find the optimum point of heat transfer coefficient by changing the protrusion height of the nozzles from 50 mm−Z to 200 mm−Z, and, based on the experiment, proposed a cooling apparatus having the most efficient cooling capacity at that time as a cooling apparatus used in the cooling zone of a continues annealing furnace. As a result of the development of the cooling apparatus, it was made possible to raise the heat transfer coefficient, which had usually been 100 Kcal/m2·hr·° C., to 400 Kcal/m2·hr·° C.
A further enhancement of the cooling rate was required thereafter, but there was a limit in the enhancement of the cooling rate as far as conventional apparatuses were concerned, wherein an atmosphere gas of 95% or so of N2 mixed with 5% or so of H2 was circulated, in most cases, as a cooling medium.
The use of hydrogen gas as the cooling medium was proposed for the purpose of solving the problem. It had long been known that cooling capacity could be improved by using hydrogen gas, but this had not been commercially applied before owing to the dangerous nature of hydrogen gas.
Japanese Unexamined Patent Publication No. H9-235626 discloses a technology to realize rapid cooling by raising the concentration of hydrogen gas. This is a technology to raise the cooling rate by controlling the hydrogen concentration in a cooling gas to 30 to 60% and its temperature to 30 to 150° C. and blowing the gas onto a steel strip at a blowing speed of 100 to 150 m/sec. in a rapid cooling zone. Further, to achieve a desired cooling rate, the distance from the steel strip surface to the tips of the protruding nozzles, each having a round blowing hole, is set at 70 mm or less.
A technology for using hydrogen gas as the cooling medium has thus been proposed concretely, and its commercial application is imminent.